The recording and processing of bioelectric signals is an important monitoring technique for many scientific and medical procedures. Bioelectric signals refer to electrical signals that are generated in the bodies of living organisms in general, and to animals and humans in particular. Monitoring equipment records the electrical signals for display and analysis in automated systems or by scientists and medical professionals. Many existing systems that monitor electrical signals in a living subject perform amplification and filtering of the analog electrical signals that are measured in the subject prior to generating digital signal data using, for example, an analog to digital converter (ADC). Numerous data processing devices including personal computers (PCs) and specialized digital signal processor (DSP) devices perform additional processing of the digital signal data.
In a living organism, such as a human being, the ionic and chemical signaling mechanisms between cells in the body form the basis for the generation of bioelectric biopotentials. The biopotentials propagate and transmit information across electrically active tissues and between different locales in the body. The changes in biopotential occur due to the flow of different types of ions into and out of cells. In one example, the biopotential mechanisms include ionic gradients and voltage-gated sodium and potassium ion channels in different tissues in the body in order to convey information to different locations or tissues. The changes in biopotential occur at the cellular level when changes in trans-membrane potentials of a cell lead to the opening of sodium and potassium channels to permit ions into and out of the cell along electrochemical gradients.
The gradients occur because the concentration of sodium outside the cell is nearly 10 times higher, specifically 145 mM:15 mM ([out]:[in]), while the concentration of potassium inside the cell is nearly 27 times higher, specifically 120 mM:4.5 mM ([in]:[out]). The ionic concentration differential leads to a standing trans-membrane biopotential that can be momentarily discharged to transmit information from one part of the cell to another. The momentary discharge is manifested locally through the opening of voltage-gated ion-selective channels. As the channels open, the ions move down concentration gradients from regions of high concentration to low concentration. Thus, the sodium flows into the cell and potassium flows out of the cell. Specifically, once the trans-membrane potential is raised from −90 mV to about −70 mV due to a depolarizing stimulus, the sodium ion channels are activated and begin to let sodium ions into the cell. This migration causes the trans-membrane potential to become less negative and rapidly depolarizes the cell. As the cell depolarizes the potassium ion channels are activated and begin to release potassium ions into the interstitial space which slows the depolarization. As the depolarization slows down, the sodium ion channels close causing the depolarization to reverse since the potassium ion channels are still open and are releasing potassium ions. As the trans-membrane potential returns to −90 mV, the potassium ion channels gradually close causing the resting potential to be achieved. While at rest, the sodium-potassium pump actively restores the gradients for the next activation.
In different organisms, such as humans, the effects of the biopotentials in different types of nerve activity produce various types of electrical signals with different amplitudes, frequencies, and durations. Different types of bioelectrical signals are measured for different types of tissue and biological functions in the body. Common types of bioelectrical signals that are monitored in humans include electrocardiograms (ECG), electroencephalograms (EEG), electromyograms (EMG), and electroneurograms (ENG). The ECG signals originate in the heart. The ECG has frequency content that covers a wide range. The ECG includes three main waveforms: the P-wave, the QRS complex, and the T-wave. The P- and T-wave are comprised of lower frequency content in the tens of Hertz (Hz) while the QRS complex is a higher-frequency event that is centered near 150 Hz. Additionally, the amplitude of the QRS is approximately several millivolts (mV) and is larger than that of the P- or T-wave. The EEG signals originate in neurons inside the central nervous system. EEG signals generally contain low frequency information in a range of approximately 0.2 Hz to approximately 50 Hz and usually have amplitudes in the low microvolt range. The EMG signals originate in muscle fibers in the body. The EMG signals recorded during muscle contraction span a frequency range of 10 Hz to 2 kHz and vary in amplitude depending on their recording location but can range from a few microvolts to a few millivolts. The ENG signals originate from nerve axons in the peripheral nervous system. ENG signals usually have maximum amplitudes of several microvolts and the duration is approximately 1 ms but can vary widely. Its short duration seems to coincide with its high frequency content, which ranges from several hundreds of Hertz to tens of kilohertz.
During monitoring, another type of signal, the electrode-interface potential (EIP), is generated due to a voltage potential between one or more electrodes that are used to monitor bioelectric signals and the tissue in the subject around the electrodes. The EIP is not a bioelectric signal of interest for monitoring the subject, but is often present as a component in monitoring the other forms of bioelectric signals. In some monitoring conditions, the EIP signal has an amplitude that is larger than the amplitudes of the bioelectric signals that are generated in the subject.
The following table lists properties of the EIP, ECG, EEG, EMG, and ENG signals described above as observed in many human patients:
FrequencyPoint ofBandwidthBioelectric SignalOrigin(Hz)AmplitudeDurationElectrointerfaceTissue- 0-0.2 HzLow millivolts (mV) toIndeterminatePotentialElectrodevolts (V)(occurs during(EIP)Interfacethe entiremonitoringperiod)ElectrocardiogramHeart0.2-200 Hz 2-3 mV (QRS complex)Up to 100ms(ECG)ElectroencephalogramCentral0.2-50 Hz10-300 μV5-10ms(EEG)NervoussystemElectromyogramMuscles10 Hz-2 kHz5 μV-20 mV (surface);2ms(EMG)50-1000 μV (invasive)ElectroneurogramPeripheral100 Hz-10 kHzLow microvolts μV1ms(ENG)Nerves
Drawbacks of present bioelectrical signal monitoring systems include the introduction of irreversible distortion to the electrical signals and the difficulty that present monitoring systems have with simultaneous monitoring of bioelectrical signals over a wide range of frequencies from direct-current (DC) to high-frequency signals. The distortion is introduced by the amplifiers and filters that are used to boost the bioelectrical signals and reduce noise, but the distortion can hinder analyses that rely on the unique morphological differences between bioelectric signal events. To generate amplified signals that minimize the distortion, existing monitoring systems are typically limited to monitoring a relatively narrow frequency range and are often only effective for monitoring the bioelectrical signals for a single type of nerve activity. The simultaneous monitoring of multiple types of nerve activity requires the use of multiple monitoring systems that are each configured to monitor different sets of bioelectrical signals in the subject. The limitations in current monitoring systems increase the difficulty in performing simultaneous monitoring of multiple types of electrical activity in a subject. Consequently, improved systems for measuring electrical signals corresponding to multiple types of nerve activity in a subject would be beneficial.